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<div class="titlepage"><div><div><h3 class="title">
<a name="math_toolkit.pol_ref.assert_undefined"></a><a class="link" href="assert_undefined.html" title="Mathematically Undefined Function Policies">Mathematically
      Undefined Function Policies</a>
</h3></div></div></div>
<p>
        There are some functions that are generic (they are present for all the statistical
        distributions supported) but which may be mathematically undefined for certain
        distributions, but defined for others.
      </p>
<p>
        For example, the Cauchy distribution does not have a meaningful mean, so
        what should
      </p>
<pre class="programlisting"><span class="identifier">mean</span><span class="special">(</span><span class="identifier">cauchy</span><span class="special">&lt;&gt;());</span>
</pre>
<p>
        return, and should such an expression even compile at all?
      </p>
<p>
        The default behaviour is for all such functions to not compile at all - in
        fact they will raise a <a href="http://www.boost.org/libs/static_assert/index.html" target="_top">static
        assertion</a> - but by changing the policy we can have them return the
        result of a domain error instead (which may well throw an exception, depending
        on the error handling policy).
      </p>
<p>
        This behaviour is controlled by the <code class="computeroutput"><span class="identifier">assert_undefined</span><span class="special">&lt;&gt;</span></code> policy:
      </p>
<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">boost</span><span class="special">{</span> <span class="keyword">namespace</span> <span class="identifier">math</span><span class="special">{</span> <span class="keyword">namespace</span> <span class="identifier">policies</span> <span class="special">{</span>

<span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">bool</span> <span class="identifier">b</span><span class="special">&gt;</span>
<span class="keyword">class</span> <span class="identifier">assert_undefined</span><span class="special">;</span>

<span class="special">}}}</span> <span class="comment">//namespaces</span>
</pre>
<p>
        For example:
      </p>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">math</span><span class="special">/</span><span class="identifier">distributions</span><span class="special">/</span><span class="identifier">cauchy</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>

<span class="keyword">using</span> <span class="keyword">namespace</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">math</span><span class="special">::</span><span class="identifier">policies</span><span class="special">;</span>
<span class="keyword">using</span> <span class="keyword">namespace</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">math</span><span class="special">;</span>

<span class="comment">// This will not compile, cauchy has no mean!</span>
<span class="keyword">double</span> <span class="identifier">m1</span> <span class="special">=</span> <span class="identifier">mean</span><span class="special">(</span><span class="identifier">cauchy</span><span class="special">());</span>

<span class="comment">// This will compile, but raises a domain error!</span>
<span class="keyword">double</span> <span class="identifier">m2</span> <span class="special">=</span> <span class="identifier">mean</span><span class="special">(</span><span class="identifier">cauchy_distribution</span><span class="special">&lt;</span><span class="keyword">double</span><span class="special">,</span> <span class="identifier">policy</span><span class="special">&lt;</span><span class="identifier">assert_undefined</span><span class="special">&lt;</span><span class="keyword">false</span><span class="special">&gt;</span> <span class="special">&gt;</span> <span class="special">&gt;());</span>
</pre>
<p>
        <code class="computeroutput"><span class="identifier">policy</span><span class="special">&lt;</span><span class="identifier">assert_undefined</span><span class="special">&lt;</span><span class="keyword">false</span><span class="special">&gt;</span></code>
        behaviour can also be obtained by defining the macro
      </p>
<pre class="programlisting"><span class="preprocessor">#define</span> <span class="identifier">BOOST_MATH_ASSERT_UNDEFINED_POLICY</span> <span class="keyword">false</span>
</pre>
<p>
        at the head of the file - see <a class="link" href="policy_defaults.html" title="Using Macros to Change the Policy Defaults">Using
        Macros to Change the Policy Defaults</a>.
      </p>
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